Photo: Nan Palmero/Flickr
That more trees in urban areas improve air quality is pretty firmly established at this point. What a new study does, however, is quantify this reduction in pollution in terms of lives saved each year.
Research conducted by the Davey Institute and the U.S. Forest Service shows that by reducing fine particulate air pollution (PM 2.5) the untimely demise of eight people is avoided each year. That's notably higher than the national average, however, where urban trees save on average one life annually.
That may not sound like much—frankly, it isn't—but it's an important measurement of how something that we take often can take for granted such as trees has notable affects on life, in often unexpected ways.
The Forest Service's Michael T. Rains explains:
More than 80 percent of Americans live in urban areas containing over 100 million acres of trees and forests. This research clearly illustrates that America's urban forests are critical capital investments helping produce clean air and water, reduce energy costs, and making cities more livable.
What's going on is this: After looking at trees in Atlanta, Baltimore, Boston, Chicago, Los Angeles, Minneapolis, New York City, Philadelphia, San Francisco, and Syracuse, the scientists found the annual removal of fine particulate matter varied from 4.7 metric tons in Syracuse to 64.5 metric tons in Atlanta. This correlates to one person's life per 365,000 people saved each year in Atlanta, to one life saved for every 1.35 million people in San Francisco.
Fine particulate air pollution can cause (in addition to premature death): Inflammation of the lungs, accelerate hardening of the arteries, and altered heart function.
Looking at the ability of trees to reduce urban air pollution more broadly, last summer researchers discovered that the ability of trees and all sorts of plants to reduce particulate air pollution as well as nitrogen dioxide is even greater than commonly thought.
The study concluded that judicious placement of grass, climbing ivy and other plants in urban canyons can reduce the concentration at street level of NO2 by as much as 40 percent and PM by 60 percent, much more than previously believed. The authors even suggest building plant-covered "green billboards" in these urban canyons to increase the amount of foliage. Trees were also shown to be effective, but only if care is taken to avoid trapping pollutants beneath their crowns.
In terms of oxygenated volatile organic compounds, trees do an amazing job at taking them out of the atmosphere—with some species actually increasing the amount of chemicals they remove when the tree is stressed or wounded.
Plants can produce chemicals to protect themselves from irritants and repel invaders such as insects, much as a human body may increase its production of white blood cells in reaction to an infection. But these chemicals, if produced in enough quantity, can become toxic to the plant itself. In order to metabolize these chemicals, the plants start increasing the levels of enzymes that transform the chemicals into less toxic substances. At the same time, as it turns out, the plant draws down more oVOCs, which can be metabolized by the enzymes.
If that weren't enough—cleaning up the air, saving lives, and all that—urban trees make us happier, as well: cooling the air, encouraging us to spend more time outdoors, and reducing stress.